In a conventional optical disc apparatus, for reproducing a signal, an optical disc which is an information carrier is irradiated with a relatively weak optical beam of a constant amount of light, and the light reflected from the optical disc is modulated to be stronger or weaker and is then detected. For recording a signal, the intensity of the optical beam is modulated to be stronger or weaker in accordance with a signal to be recorded and information is written on a recording material film on an optical disc.
On an optical disc for reproduction only, information is previously recorded in a spiral pattern with pits. An optical disc for both recording and reproduction is produced by forming a film of a material which can be optically recorded or reproduced on a surface of a base material having a track of a convex-concave structure in a spiral pattern by a method such as evaporation.
For recording information on or reproducing information recorded on an optical disc, focus control and tracking control are required. Focus control is control of an optical head along a line normal to a surface of the optical disc (hereinafter, referred to as a focus direction) so as to have a optical beam to always be in a predetermined convergence state on a recording material film. Tracking control is control of an optical head in a radial direction of an optical disc (hereinafter, referred to as a tracking direction) so as to have a optical beam to always be on a predetermined track.
An operation of a conventional optical disc apparatus will be described with reference to FIGS. 9 through 11.
FIG. 9 is a block diagram of the optical disc apparatus.
FIG. 10(a) shows an example of a signal output from an FE generator 20. FIG. 10(b) shows an example of a signal output from an AS generator 21. In FIG. 10, a horizontal axis indicates a position of a focus of an optical beam in the focus direction with respect to an optical disc 1.
FIG. 11(a) shows an example of a signal output from the FE generator 20 when the optical disc 1 is either in a recorded state or an unrecorded state. FIG. 11(b) shows an example of a signal output from the AS generator 21 when the optical disc 1 is either in a recorded state or an unrecorded state. FIG. 11(c) shows an example of a signal output from a normalization operator 22. In FIG. 11, a horizontal axis indicates a position of a focus of an optical beam in the focus direction with respect to the optical disc 1.
As shown in FIG. 9, the unit for detecting focus error is an FE generator 20. The unit for detecting light amount is an AS generator 21. The unit for normalizing focus error is a normalization operator 22. The unit for focus control includes an Fc filter 23 and a selector 24. The unit for measuring amplitude is an FE amplitude measuring device 30. The unit for calculating gain is a gain operator 32.
An optical head 10 includes a semiconductor laser 11, a condensing lens 13, a beam splitter 12, a focus actuator 14, a tracking actuator 15, and a photodetector 17.
An optical beam generated from the semiconductor laser 11 passes through the beam splitter 12 and is converged on the optical disc 1 having a disc shape by the condensing lens 13. The optical beam reflected off the optical disc 1 passes through the condensing lens 13 again, and is reflected off the beams splitter 12 to impinge upon the photodetector 17. The condensing lens 13 is supported by an elastic body (not shown). When a current flows through the focus actuator 14, the condensing lens 13 is moved in the focus direction by an electromagnetic force. When a current flows through the tracking actuator 15, the condensing lens 13 is moved in the tracking direction by an electromagnetic force. The photodetector 17 sends the detected light amount signal respectively to the FE generator 20 and the AS generator 21.
From the light amount signal of the photodetector 17, the FE generator 20 calculates an error signal indicating a convergence state of the optical beam on an information surface of the optical disc 1, i.e., an error signal which corresponds to a shift in a position of the focus of the optical beam with respect to the information surface of the optical disc 1 (hereinafter, referred to as an FE signal). The FE generator 20 sends the FE signal to the normalization operator 22 and the FE amplitude measuring device 30. The AS generator 21 detects an amount of light reflected off the optical disc 1 based on the signal from the photodetector 17 and sends the detected amount of light to the normalization operator 22 and an AS level measuring device 31.
The normalization operator 22 divides the signal from the FE generator 20 by the signal from the AS generator 21 and multiply the result by an internal gain. The normalized signal is then sent to the Fc filter 23. The Fc filter 23 generates a driving signal for focus control based on the signal from the normalization operator 22 and sends the signal to the focus actuator 14 via the selector 24. A search driving generator 33 generates a search driving signal for moving the condensing lens 13 vertically in order to measure an amplitude of the FE signal and a level of the AS signal and sends the signal to the focus actuator 14 via the selector 24.
The FE amplitude measuring device 30 measures the largest value and the smallest value of the FE signal from the FE signal generator while the condensing lens 13 is being moved by the search driving signal from the search driving generator 33. In this way, the amplitude of the FE signal from the FE generator 20 is measured and the measured value is sent to the gain operator 32. The AS level measuring device 31 measures the largest value of the signal from the AS generator 21 while the condensing lens 13 is being moved by the search driving signal from the search driving generator 33. In this way, the level of the AS signal is measured, and the measured value is sent to the gain operator 32.
The gain operator 32 multiplies a preset FE target amplitude by the measured value from the AS level measuring device 31, and then divides the result by the measured value from the FE amplitude measuring device 30. In this way, the gain value is obtained. The gain value is set as an internal gain of the normalization operator 22.
Conventionally, there have been some types of optical discs which include a plurality of information surfaces. For example, some DVDs have one layer and others have two layers. If the optical disc 1 includes a plurality of information surfaces, when the condensing lens 13 is moved vertically for search, an FE signal and an AS signal are generated for each of the information surfaces as shown in FIG. 10. The reflectances of two information surfaces are different. Thus, there are variances in the amplitudes of the generated FE signal and the level of the AS signal between the information surfaces. These variances correspond to the reflectances of the information surfaces. For an information surface with a high reflectance, both the amplitude of the FE signal and the level of the AS signal are large (see, for example, left-hand sides of FIG. 10). For an information surface with a low reflectance, both the amplitude of the FE signal and the level of the AS signal are small (see, for example, right-hand sides of FIG. 10).
Even on the same information surface, reflectance varies depending upon the radial position of the optical disc 1. Furthermore, even the reflectance of the information surface material itself is uniform, the reflectance become different between a recorded portion and an unrecorded portion thereof. For example, as shown in FIGS. 11a) and 11b), the amplitude property of the FE signal and the level property of the AS signal in the unrecorded state may be as represented by solid lines, but they may change into the amplitude property of the FE signal and the level property of the AS signal as represented by broken lines in the recorded state. This is because, when the information surface is recorded, the state of the film forming the information surface changes, causing the reflectance to change.
When the focus control is working with respect to the information surface, if the control gain varies for different parts of the information surfaces, the focus control may be instable. Thus, a method which enables keeping the control gain constant is desired. For example, in the optical disc apparatus shown in FIG. 9, the normalization operator 22 uses the AS signal to normalize the FE signal. Thus, the normalized FE signal which does not depend upon the reflectance change as shown in FIG. 11(c) can be obtained.
For obtaining the normalized FE signal, the normalization operator 22 divides the signal from the FE generator 20 by the signal from the AS generator 21, and then multiply the result by the internal gain. The FE signal from the FE generator 20 is normalized to a signal having a target amplitude with the internal gain of the normalization operator 22. In this way, the control gain of the focus control becomes a predetermined gain that is kept constant.
For obtaining the internal gain of the normalization operator 22, the amplitude of the FE signal from the FE generator 20 and the level of the AS signal from the AS generator 21 have to be measured. For this measurement, in the optical disc apparatus shown in FIG. 9, first the focus control is kept to be in a non-operational state, and a driving signal for moving the condensing lens 13 vertically for search is generated from the search driving generator 33 to operate the condensing lens 13 such that the focus of the optical beam passes through the information surface of the optical disc 1. At this time, the signals as shown in FIG. 10 are output from the FE generator 20 and the AS generator 21 depending upon the relative positions of the focus of the optical beam and the optical disc 1. The FE amplitude measuring device 30 finds the amplitude of the FE signal by measuring the largest value and the smallest value of the FE signal from the FE signal generator 20. The AS level measuring device 31 finds out the level of the AS signal by measuring the largest value of the AS signal from the AS generator 21. For example, for both the amplitude of the FE signal and the level of the AS signal, the values acquired from the information surface with a large reflectance (see, for example, the left-hand sides of FIG. 10) are used for the measurement. Based on the amplitude of the FE signal and the level of the AS signal measured as such, the gain operator 32 calculates the internal gain of the normalization operator 22 (see, for example, Prior Art Reference 1)
[Prior Art Reference 1] Japanese Laid-Open Publication No. 2002-373431